Many types of safety sensing devices for motors are known in the patent literature and appear on the market. The vast majority of these sense motor characteristics and conditions during operation of the motor. The protective apparatus which operates only during running of the motor cannot, however, prevent motor burnouts which occur upon initial application of electrical power to the motor windings. Such burnouts result from the degradation of insulation resistance caused by condensation and contamination. The frequency of such occurrences is a function of the relative humidity, the concentration of airborne particles and temperature changes in the operating environment. It may be readily appreciated that contaminated or rugged environments may require motor protection against insulation degradation which results in motor burnout at start-up.
Protective devices have been proposed for use with electric motors to monitor the windings resistance at rest. U.S. Pat. No. 3,656,136 shows an electric motor with a safety sensing device. Here, a small current is applied to one winding of a motor, and the current level is monitored. Should it reach too high a level, indicating a low resistance leakage path to ground, an alarm indication is provided and a relay is activated preventing energization of the motor.
Another type of protective device is described in U.S. Pat. No. 3,611,036. Two motor terminals are connected to two input power lines through diodes in series with neon lamps and current limiting resistors. A ground in the motor produces a current flow through one or both lamps. A photosensitive resistor, in response to light from the lamps, activates a relay control circuit to open the input line to the motor, so that the motor cannot be turned on.